Inter alia from U.S. Pat. No. 3,986,097 (see, for example FIG. 1) is known a switched mode power supply working as a flyback converter. Here too, first periods of time in which a primary current flows through the primary winding of the transformer alternate with second periods of time in which a secondary current flows through the secondary winding of the transformer. The primary current linearly rises from a minimum value to a maximum value in the first periods of time. The secondary current linearly drops from a maximum value to a minimum value in the second periods of time. In ranges where the primary current is positive, energy is transported from the primary side to the transformer. At the beginning of the second periods of time, this energy is then transported by the transformer on the secondary side in the direction of the output of the switched mode power supply, while the secondary current is positive. After the secondary current has reached the zero value, it becomes negative and further drops to the minimum value of the secondary current. In this range the part of the energy supplied by the transformer on the secondary side, which energy is not necessary for powering the load, is reflected back into the transformer. By varying the minimum value of the secondary current, the energy reflected back to the transformer by the secondary side is adjusted. The value of the respective minimum secondary current is a contributory factor for determining the minimum value of the primary current of the next first time slot in accordance with the translation ratio of the transformer. The minimum value of the primary current now determines the range in which the primary current is negative and thus the amount of energy that is returned by the transformer in the direction of the input of the switched mode power supply in the respective first time slot.
In EP 0 336 725 B1 is described a switched mode power supply in which a first switching device is connected in series with the primary winding, which switching device is controlled by a control circuit. The control circuit evaluates a falling auxiliary voltage on a primary-side auxiliary winding of the transformer. The first switching device is controlled by the control circuit in such a way that the length of the periods of time in which the first switching device is switched on, i.e. is closed and thus conductive, has a fixed value. The control circuit switches on the primary-side switching device, i.e. brings it to the closed and thus conductive state, as soon as the voltage on the auxiliary winding becomes positive. The control circuit controls the switching device so that the length of the periods of time in which the first switching device is switched on has a fixed value. The first switching device thus has a constant switch-on time. A second switching device connected in series with the secondary winding of the transformer is controlled by a control unit. This unit detects the output voltage of the switched mode power supply, compares the detected voltage to an internal reference voltage and thus controls, in dependence on the predefinable value of the reference voltage, the length of the periods of time in which the second switching device is switched on, and, accordingly, the amount of energy reflected back from the secondary side to the transformer and to the primary side. With an increasing load on the output of the switched mode power supply, the amount of reflected energy decreases. With a decreasing load, the amount of reflected energy increases. The control unit thus controls the output voltage of the switched mode power supply to a constant value. In the switched mode power supply it is no longer necessary to transmit a control signal describing the deviation of the output voltage from a setting value to the primary side via a DC isolation path, for example, by means of an optocoupler. On the other hand, only in the ranges with a negative primary current is a zero-voltage switching of the primary-side switching device possible, because then the diode connected in parallel to the switching device is conductive. With rather large output powers, the ranges in which the primary current is negative are reduced and the ranges in which a positive primary current flows are lengthened accordingly. Especially with large output powers for which the first switching device has the largest ohmic losses, this may lead to the fact that in first periods of time no negative primary current flows any longer and, accordingly, no zero-voltage switching of the first switching device is guaranteed any longer.